BANGKOK, Thailand — U.S. Assistant Secretary of Defense Richard L. Armitage concluded talks today with top Vietnamese officials on missing American servicemen, then blasted allegations that the U.S. government has covered up reports of Americans still being held prisoner in Indochina.

He told a news conference that such allegations are harming official efforts to resolve the fates of about 1,800 Americans still listed as missing in action in the Vietnam War, which ended in 1973.

Armitage, who met Vietnamese Foreign Minister Nguyen Co Thach during two days of discussions in Hanoi on MIAs, said that reports by some private Americans of live American prisoners were suspect and that the motives for issuing such reports were questionable.

Armitage said Vietnamese authorities agreed to investigate the question of American prisoners. But he said that so far the U.S. government has no proof that there is any truth to reports of American prisoners being sighted alive.

Vietnam announced today that it has gathered 50 new reports on American MIAs and hopes to resolve the problem by 1988.

Armitage said U.S. and Vietnamese officials will discuss the technical aspects of the 50 new reports in Hanoi next month. He said Vietnam denied holding any live Americans.

However, Ann Mills Griffiths, executive director of the National League of POW-MIA Families, who accompanied the U.S. mission to Hanoi, told the same news conference in Bangkok that information received by her organization indicated that some Americans are being held alive in Vietnam and Laos.

6 Accuse Administration

The highest-level U.S. government delegation to Vietnam since the war ended included Assistant Secretary of State Paul D. Wolfowitz and National Security Council staff member Richard K. Childress.

Six Americans, some of whom said they personally saw American prisoners of war as recently as four months ago in Vietnam and Laos, on Monday accused the Reagan Administration of quashing witness accounts.

They filed an affidavit in the U.S. District Court in Fayetteville, N.C.